Pin/Parent Vertex of OBJ1 to Vertex of OBJ2
if you select two objects, you can hook a vertex from the last selected (after entering edit mode) to "selected object", ie to the last object selected (to its origin). You could move the origin to a vertex in this last object... but it's not ideal 

Or, you can do this, perhaps better:


from the object 1 (the one that has the vertex you wish to pin to), select the vertex and move the 3d cursor there (Shift+S > "cursor to selected")
then add a new empty, and move it to the 3d cursor (Shift+S > "selected to cursor")
then parent the empty to the object 1
then selected the empty and then the object 2 with the vertex you wish to pin to the object 1 vertex
enter edit mode, and select the object 2 vertex to be pinned 
from mesh > vertices > hooks > "hook to selected object" (the empty)




if I got it well...